WASHINGTON (Reuters) – U.S. taxpayers owe an estimated $385 billion in unpaid taxes for 2006, up about a third from the “tax gap” five years before, the Internal Revenue Service (IRS) said.

Despite the wider shortfall between 2001 and 2006, voluntary compliance remained steady at about 83 percent for both years, the agency said on Friday in its five-year update on the amount of taxes that are never paid.

“On a relative basis, the tax gap is largely in line with the growth in total tax liabilities,” the IRS said in a statement.

“In addition, some growth in the tax gap estimate is attributed to better data and improved estimation methods.”

The gross tax gap, or the amount that was not paid on time, was estimated at $450 billion in 2006, up from $345 billion in 2001.

The net tax gap, or the amount that was never paid, was $385 billion in 2006, compared with $290 billion five years before, the tax agency said.

Most of the wider tax gap over the five years was concentrated in underreporting and underpayment. The non-filing portion was largely unchanged.

Underreporting climbed to $376 billion from $285 billion, and more than a third of the growth was from corporate income taxes.

Underreporting on corporate income taxes more than doubled to $67 billion, the IRS said.